Thin polymeric films are formed of thermoplastic polymers that are made into sheets using any of a variety of processes, such as film extrusion. The polymeric films have a multitude of applications, such as packaging, magnetic media coatings, etc. The polymer may be “oriented” during formation of the film. One technique produces what is commonly referred to as “stretched polymer film,” whereby the long polymer molecules are substantially aligned in one direction. Stretching the film improves its physical properties, such as its stiffness and dimensional stability. Stretching also improves a film's optical properties and vapor-barrier effectiveness. There are other ways of orienting polymeric material. For example, photoalignment processes may be employed, whereby polymer alignment results from irradiation of a polymeric film with ultraviolet light.
Polymeric films have birefringence properties that are described in more detail below. In short, the measure of the film's birefringence properties can yield useful insights into the physical, optical, or other properties of the film. For example, the measured level of birefringence in a polymer film can be readily correlated to the extent of polymer orientation or stretch that the film possesses and, therefore, correlated to a desired physical property in the film.
Birefringence means that different linear polarizations of light travel at different speeds through a light-transmissive element such as polymeric film. Retardation or retardance represents the integrated effect of birefringence acting along the path of a light beam traversing the film. If the incident light beam is linearly polarized, two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The fundamental unit of retardance is length, such as nanometers (nm). It is frequently convenient, however, to express retardance in units of phase angle (waves, radians, or degrees), which is proportional to the retardance (nm) divided by the wavelength of the light (nm).
Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below.
The two orthogonal polarization components described above are parallel to two orthogonal axes, which are characteristic of the measured portion of the polymeric film sample and are respectively referred to as the “fast axis” and the “slow axis.” The fast axis is the axis of the polymeric film that aligns with the faster moving component of the polarized light through the sample. Therefore, a complete description of the retardance of a polymeric film sample along a given optical path requires specifying both the magnitude of the retardance and the relative angular orientation of the fast (or slow) axis of the sample. Oriented (stretched) polymeric film will have an axis that corresponds to the orientation direction of the polymers and that, in turn corresponds to either the “fast” or “slow” axis described above. This fact can be exploited for simplifying the measure of retardance in the stretched polymeric film and for quickly determining the very high levels of retardance that can be present in such films.